Coolants are an instrumental part of machining, including grinding, milling, and turning. They help extend tool life and provide an improved surface finish of the parts being machined. Understanding the role and types of coolant help you select a coolant that is the right fit for your machine and operation. By properly maintaining the concentration levels of your coolant, you extend not only the life of the coolant but also your tools and machine.
What is the Role of Coolants?
The critical functions of coolant in the machining process include:
- Reducing and removing the heat build-up in the cutting zone and workpiece
- Provides lubrication to reduce friction between the tool and removal of the chips
- Flushes away chips and small abrasive particles from the work area
- Protects against corrosion
The type of machining and materials machined determine the type of coolant to use as well as what the balance of cooling and lubrication is needed. By altering the mixing ratio or concentration of the coolant, you get a different balance of cooling and lubrication. A leaner mix provides you better cooling while a more concentrated blend gives you more lubrication.
Types of Coolants
Coolants are grouped into four main categories and have a variety of different formulations. Selecting coolant should be based on the overall performance it provides centered around your machining application and materials used.
Soluble Oils: The most common of all water-soluble cutting fluids and a great option for general purpose machining. The drawback is that they are prone to microbiological growth of fungus and bacteria if the coolant sump is not correctly maintained.
Synthetic Fluids: These types of fluids tend to be the cleanest of all cutting fluids because they contain no mineral oil and reject tramp oil. However, they provide the least lubrication.
Semi-synthetic Fluids: Considered to be the best of both worlds, they have less oil than emulsion-based fluids, a less stinky smell, and retain much of the same lubricating attributes. This makes them usable for a broader range of machining.
Straight Oils: These are not water-miscible and have a composition of a mineral or petroleum oil base and contain lubricants like vegetable oils, fats, and esters. They provide the best lubrication but have the poorest cooling characteristics.
How Machine Coolant Systems Work
During the machining process, the coolant mixture floods over the work area. This process also washes chips and particles away from the work area. Coolant collects in a sump at the bottom of the machine. The coolant is pumped out of the sump and recirculated to the work area.
Both central and single machine coolant systems need to be monitored, maintained, and adjusted. Unfortunately, small coolant systems tend to use less effective equipment for filtration and oil separation in comparison to central systems. Small systems are also susceptible to more rapid changes and greater fluctuations in concretion levels. Therefore, the coolant used in small systems needs to be more tolerant of contamination from metal shavings, tramp oils, and other materials. Not only does the coolant type play a role in extending the life of your coolant, but proper coolant management becomes even more critical.
If appropriate concentration levels of coolant are not maintained, several issues can occur. The most common problem is low concentration. If the concentration of coolant is below the machine coolant supplier’s minimum ratio, there is a risk of:
- Machine and workpiece corrosion
- Reduction in tool life
- Bacterial growth
On the other hand, if the concentration of coolant is too high, this results in:
- Lesser heat transfer
- Reduced lubrication
- Wasted concentrate
- Formation of residue that shortens tool life
- Staining of machine and machined parts
- Toxicity (skin irritation)
At the start of each day, the coolant should be checked to maintain an acceptable concentration level. Hand refractometers are a great way to check cutting and grinding fluid concentrations to maintain daily control of concentration levels. Machine coolant concentrations can change 5% to 20% every day from evaporation, splashing, misting, and dragout. Keeping a daily log of concentration levels for each machine provides an understanding of how the system is functioning and how much concentration levels change from day-to-day.
By selecting the right coolant for the type of machine and metals being machined, and by maintaining the concentration levels, you extend the life of the coolant, the tools, and your machine.